Generally, a hydraulic power transmission, such as a torque converter and a fluid coupling, is employed as a clutch assembly of a vehicle. In the case where the hydraulic power transmission is employed in the vehicle having an internal combustion engine that keeps the drive source in an operating state, even if the vehicle is stopped, the fluid supplied from the hydraulic power transmission allows a relative rotation between an input shaft of the automatic transmission and an output shaft of the engine so as to keep the operation of the drive source. The fluid of the hydraulic power transmission, thus, allows the vehicle to have a smooth take-off and power of the engine is transmitted to the input shaft of the automatic transmission.
The aforementioned hydraulic power transmission or the torque converter allows the vehicle to have a good automatic take-off performance owing to the relative rotation between the input and output members, and slip function. The required size of the hydraulic power transmission, however, becomes relatively large. This may interfere with reduction in weight and size of the vehicle. As a lock-up clutch is employed to prevent a power loss that generally occurs during power transmission through the fluid, the structure of the hydraulic power transmission may become further complicated. Even if the lock-up clutch is employed for the aforementioned reason, it cannot eliminate all the power loss.
The demand for the torque converter to increase the torque ratio or provide a shock absorbing function of the fluid upon shifting has been decreased owing to the increase in the shift speeds of the automatic transmission and further development of the continuously variable transmission (CVT). Instead the clutch assembly using no fluid has been proposed in response to the demand for reducing weight and size of the automatic transmission.
For example, Japanese Patent Laid-Open Publication No. 2001-3955 discloses that the clutch assembly has a housing connected to an internal combustion engine, where a multiple disc clutch, a damper device and a one-way clutch are housed therein. When the vehicle is stopped, the multiple disc clutch is connected by a pressure spring so as to generate a predetermined creep force. When the vehicle is about to take off, the multiple disc clutch is engaged under pressure of a piston upon supply of an oil pressure to an oil chamber. The driving force of the engine is then transmitted to the input shaft while the impact being absorbed by the damper device. Further the one-way clutch serves to prevent the vehicle from moving backward upon running uphill.
The aforementioned clutch assembly has a clutch case serving as an output member and a hub serving as an input member in the housing such that a wet-type multiple disc clutch is formed by interposing a plurality of friction plates between the clutch case and the hub. The piston that presses the clutch is fit oil tightly with the clutch case and an outer race of the one-way clutch via O-rings, respectively. An oil pressure is supplied between the outer race and the clutch case from an oil hole formed in the input shaft to the oil chamber defined by the piston and the clutch case.
The aforementioned clutch assembly has a housing that constitutes an outer frame of the clutch assembly, in which the clutch case for the clutch is provided as well as the oil chamber. The reaction force of the clutch against the pressure acts on the clutch case, and the force in the axial direction is generated between the housing and the elements housed in the clutch case. In this case, a thrust bearing and the like has to be provided for the purpose of bearing the axial force (thrust). As the oil chamber is defined by the clutch case housed in the housing and the outer race, the oil passage through which the oil pressure is supplied/discharged between the input shaft and the oil chamber requires at least two oil seals interposing the oil hole formed in the input shaft.
In the generally employed clutch assembly, the oil chamber and the clutch are provided in the clutch case housed in the housing. The structure of the clutch assembly becomes complicated because of the needs for providing the thrust bearing and the oil passage in addition to the clutch case itself, thus degrading reliability of the clutch assembly.
Additionally, the damper device is disposed on an outer periphery in the radial direction of the multiple disc clutch. As a result, the radial size of the clutch assembly is increased. Especially when two springs for the damper device are provided in the radial direction for improving the damper performance, the radial size of the clutch may further be increased, thus deteriorating ease of mounting the clutch assembly on the vehicle.